Make the partition information dialog resizable (#690542)

Make the dialog resizable, add a vertical scrollbar to the information
and messages section, and set the initial height to ensure the dialog
fits entirely on an 800x600 screen.

A default height is required because some window managers, such as
fluxbox used in GParted Live, only permit resizing the height by using
the bottom corners of the dialog.  If the dialog is too large for the
screen then the user would not be able to resize it.

Note that two default initial heights are used in an effort to minimize
the amount of extra whitespace.

Bug 690542 - Partition Information Dialog Warning not readable